Plant SpikerBox has 3 outputs:

1.) One that can stimulate plant. It is DC output that just connects 9V battery to plant directly (via relay).

2.) USB output that sends signal recorded on the plant. Voltage of the plant in respect to ground.

3.) On 3.5 mm jack we wanted to output the same signal as on USB so that it can be used via green cable on iphone/Android etc...

Since plant has very low frequency signal (1Hz or similar) audio input on iPhone and other devices will not let that low frequency signal to go inside. It filters all signals below 20Hz.
So we used amplitude modulation (AM) with carrier signal of 5kHz modulated by plant signal and output that on 3.5mm jack. 5KHz signal goes into iPhone and in software we demodulate signal (remove 5kHz carrier) to get low frequency Plant signal.
AM modulation is just multiplication of two signals: 5kHz sinus and plant voltage